The invention relates to a device for clamping relative to each other at opposite end edges two metal objects which are to be welded together at said edges, in such a way that the end edge faces running essentially parallel to each other are held at a particular distance from each other, in particular for clamping in this way two lengths of pipe which are to be welded together at the head ends, said device comprising a draw bolt provided with an external screw thread, screwed onto it an element forming a tension nut, and a bridge piece which has an opening through which the draw bolt passes with a slight clearance and a contact face which is or can be brought into contact with a contact face on the bottom side of the tension nut element, and which has at its bottom side engaging parts which lie in one flat plane and can be engaged with one of the sides of the objects to be welded together, on either side of the space between the said end edge faces of these objects, the draw bolt at one end bearing a relatively thin part with a clamping plate or similar element, which has, a top edge facing the tension nut element and forming a radially extending shoulder, which can be engaged with the other sides of the objects to be welded together, on either side of the space between the said end edge faces thereof. Such a device is known from U.S. Pat. No. 4,175,735.
In the use of such a device for, for example, welding together lengths of pipe, in which at least three of these devices are used, these devices are each disposed between the lengths of pipe in such a way that the clamping plate comes to rest inside the lengths of pipe, at right angles to the opposite end faces thereof, and these end faces rest on either side against the relatively thin part. If the bridge piece with the engaging parts thereof is then brought to rest on either side of the space between the end faces against the outsides of the lengths of pipe, during or following which the tension nut is tightened, the walls of the lengths of pipe are clamped between the shoulder of the clamping plate resting against the insides and the engaging parts of the bridge piece resting against the outsides of the lengths of pipes, so that the end faces of the lengths of pipe come to rest running virtually parallel precisely opposite each other, with a space between them which determines the weld to be made, and the width of which is essentially equal to the thickness of the relatively thin part.
When a number of first welds have been made in the peripheral parts of the space between the end faces of the lengths of pipe, situated between the devices thus disposed, the devices are removed and the welding together of the lengths of pipe completed. On removal of each device, the tension nut is loosened and the clamping plate turned through 90 degrees, so that the clamping plate finishes parallel to the end faces of the lengths of pipe, so that this plate can be pulled out.
The known device in which said relatively thin part is formed by a neck part which on its free end bears the clamping plate has the disadvantage that, when the tension nut is tightened, the torque exerted through the screw threads on the draw bolt is transferred to the thin and thereby breakable neck part of the draw bolt, unless the handle provided at the upper end of the draw bolt is kept absolutely motionless as tension is applied. This is difficult to achieve, which means that this neck part can easily break during tightening of the tension nut. Besides, this neck part is generally subjected to bending stress in the process, since on tightening of the tension nut the insides of the lengths of pipe are taken, through the shoulders of the clamping plate, to a position in line with each other, where the shoulders can be unevenly loaded.